0-Hydroxybutyric acid dehydrogenase

A /3-hydroxybutyric acid dehydrogenase that uses DPN has been known for many years. It is specific for i/-/3-hydroxybutyrate, and does not act with the CoA derivative. The original description of the CoA-requiring system included a specific requirement for D-hydroxy acids. Later a racemase that converts ii-/3-hydroxy CoA compounds to d-was reported, but it appears that the racemase is the resultant of an D-dehydrogenase together with the previously described D-enzyme. Racemization requires DPN. ... 

E. coli C. Kluyveri 4-hydroxy-butyryl-CoA dehydrogenase and R. eutropha phaC P(4HB) 4-hydroxybutyric acid and glucose 68... 

Succinic semialdehyde dehydrogenase deficiency. Patients have mental retardation, cerebellar disease, and hypotonia. They excrete large amounts of both succinic semialdehyde and 4-hydroxybutyric acid. There is no known therapy. 

Figure 12-4. Gamma-aminobutyric acid metabolic interactions. GA = glutaminase GABA = y-aminobutyric acid GABA-T = GABA a-oxaloglutarate transaminase GAD = glutamic acid decarboxylase GS = glutamic synthetase NAD+ = nicotinamide adenine dinucleotide PP = pyridoxal phosphate (vitamin B6) SSA = succinic semialdehyde SSADH = succinic semialdehyde dehydrogenase GHB = y-hydroxybutyric acid GBL = y-butyrolactone.

Dichlorophenoxy)butyric acid is converted in the presence of ATP into dichlorophenoxybutyryl coenzyme A. This acyl-CoA is converted by the electron acceptor flavine adenine dinucleotide (FAD) into dichlorophenoxycrotonyl-CoA. One carbon atom of the unsaturated bond is hydroxilated and dichlorophenoxy- -hydroxybutyric acid-CoA is formed. In certain plants possessing specific /3-oxidase enzyme systems, -ketobutyric acid-CoA is formed from this intermediate compound by the mediation of NAD and NADH in a reaction catalysed by -hydroxyacyl-CoA dehydrogenase. This compound is decomposed by hydrolysis into 2,4-D and acetyl-CoA. 

The metabolic pathways involved in the synthesis of P(3HB-co-4HB) from 4-hydroxybutyric acid are shown in Fig. 2.4. Transferase or thiokinase catalyzes the conversion of 4-hydroxybutyric acid into 4HB-CoA, which is then used as the substrate by the PHA synthase in the polymerization reaction. The catabolism of 4-hydroxybutyric acid also leads to the formation of intermediates such as 3-hydroxybutyryl-CoA, resulting in the accumulation of P(3HB-co-4HB) copolymer. The main catabolic pathway for 4HB is probably via succinic acid semialdehyde and succinic acid pathways, which are catalyzed by 4HB dehydrogenase and succinic acid semialdehyde dehydrogenase (Valentin et al. 1995 Lutke-Eversloh and Steinbiichel 1999). All precursor substrates for the generation of 4HB monomers are first converted into 4HB-CoA, which is the immediate substrate for PHA synthase. 

The activity of hydroxymethylglutarate CoA reductase, which produces mevalonic acid—a precursor of cholesterol—was unchanged in diabetic rats. The observations made on diabetic rats contrast with those made in fasted animals in which ketosis is likely to result from activation of the hydroxymethylglutarate CoA shunt pathway, probably due to decreased activity of the hydroxymethylglutarate CoA reductase. In the presence of NADH and a specific mitochondrial dehydrogenase, acetoacetic acid is reduced to yield j8-hydroxybutyric acid, one of the ketone bodies that is excreted in the urine in ketosis. In fact, D-jS-hydroxy-butyric acid represents 50-75% of the blood content of ketone bodies. Therefore, hydroxybutyric acid metabolism assumes a particular importance. 

The /3-hydroxyisobutyrate dehydrogenase from pig kidney has been purified about 200-fold by Robinson and Coon (88) employing the usual procedures for protein fractionation. The enzyme is present in kidney, liver, and heart of vertebrates and in a number of microorganisms. The most active somrce observed was rabbit liver. The dehydrogenase has no activity on 3-hydroxybutyric acid or the CoA ester. 

The rate of mitochondrial oxidations and ATP synthesis is continually adjusted to the needs of the cell (see reviews by Brand and Murphy 1987 Brown, 1992). Physical activity and the nutritional and endocrine states determine which substrates are oxidized by skeletal muscle. Insulin increases the utilization of glucose by promoting its uptake by muscle and by decreasing the availability of free long-chain fatty acids, and of acetoacetate and 3-hydroxybutyrate formed by fatty acid oxidation in the liver, secondary to decreased lipolysis in adipose tissue. Product inhibition of pyruvate dehydrogenase by NADH and acetyl-CoA formed by fatty acid oxidation decreases glucose oxidation in muscle. 

Fig. 10. A plot of the maximal relative activity (uRmax) of /3 hydroxybutyrate dehydrogenase-lecithin mixtures versus the number of carbon atoms in the saturated fatty acid side chains of the lecithins.

Phospholipid vesicles (and bilayers) composed of phospholipids with well-defined fatty acid side chains undergo a sharp transition from a crystallinelike state to an amorphous state as the temperature is raised.107 The transition temperature depends on the nature of the fatty acid side chains. For example, for C12 saturated fatty acid chains on lecithin the transition temperature is 0° and for C18 saturated fatty acid chains it is 58°C for unsaturated lecithins the transition temperature is below zero.107 For real membranes sharp phase transitions are not observed, because of the heterogeneous composition of the membrane. In the case of /3 hydroxybutyrate dehydrogenase, the enzymic activity apparently is not influenced by this phase transition as judged by the temperature dependence of the reaction rate. However, for some membrane-bound proteins, a plot of the reaction rate versus the reciprocal temperature... 

Interconversion between ACAC and is dependent upon the NADiNADH ratio. Hydroxybutyrate dehydrogenase (HBDH) is localised mainly in the mitochondria. During fasting, fatty acids are transported to the liver to undergo beta oxidation. 

In extraliepatic tissues, d-/3-hydroxybutyrate is oxidized to acetoacetate by o-/3-hydroxybutyrate dehydrogenase (Fig. 17-19). The acetoacetate is activated to its coenzyme A ester by transfer of CoA from suc-cinyl-CoA, an intermediate of the citric acid cycle (see Fig. 16-7), in a reaction catalyzed by P-ketoacyl-CoA transferase. The acetoacetyl-CoA is then cleaved by thiolase to yield two acetyl-CoAs, which enter the citric acid cycle. Thus the ketone bodies are used as fuels. 

Somewhat surprisingly, within the mitochondria the ratio [NAD+]/[NADH] is 100 times lower than in the cytoplasm. Even though mitochondria are the site of oxidation of NADH to NAD+, the intense catabolic activity occurring in the (3 oxidation pathway and the citric acid cycle ensure extremely rapid production of NADH. Furthermore, the reduction state of NAD is apparently buffered by the low potential of the (3-hydroxybutyrate-acetoacetate couple (Chapter 18, Section C,2). Mitochondrial pyridine nucleotides also appear to be at equilibrium with glutamate dehydrogenase.169... 

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